A U-shaped enclosure in the wet pasture of Ballyvouden, County Limerick, belongs firmly to the second category. Roughly twenty-two metres in external diameter, it has never appeared on any of the historic Ordnance Survey Ireland maps that record so much else in this part of the country, and by the time Google Earth photographed the area in November 2018, it had vanished from view again entirely. Its existence is real, but its visibility, apparently, is not guaranteed.

The enclosure was first formally identified during the Bruff aerial photographic survey in 1986, when a low-altitude pass captured it clearly enough to be catalogued as reference Bruff 30.3. Aerial archaeology of this kind works by reading crop marks or soil discolouration that surface features no longer betray, and the Ballyvouden enclosure has proven a good example of how much can be missed at ground level. Later surveys added to the picture: the ASI aerial photographic survey of 23 August 2000 recorded it as faintly visible looking southward, and orthoimages gathered between 2005 and 2013 by both OSi and Digital Globe confirm the outline under the right conditions. The site sits 265 metres south of the Glenatrahaun Stream and the townland boundary with Ballynagally, in company with a ring-barrow, a burial monument of raised earthen construction, located just 55 metres to the north, and a second enclosure abutting it to the west. That clustering of features suggests this corner of County Limerick was once a more organised and inhabited landscape than the present pasture implies.

The site lies in working farmland and there is no formal public access. Because the enclosure is not visible as any kind of upstanding earthwork, a visit without specialist equipment or prior research would yield little at ground level. The feature is best appreciated through the archival aerial images rather than in person, and anyone with a serious interest would do well to consult the records compiled by Edmond O'Donovan and held through the Archaeological Survey of Ireland. The surrounding fields are notably wet, sitting in low-lying pasture, so the ground conditions alone make casual exploration impractical for much of the year.